Processes for preparing benzophenone-azines by oxidation of corresponding benzophenone-imines are conventionally known as disclosed, e.g., in U.S. Pat. No. 2,870,206 in which benzophenone-imines are contacted with molecular oxygen to produce benzophenone-azines. However, these conventional processes have technical and economical problems awaiting solution and have not yet been carried out on a large commercial scale. A particularly serious technical problem is insolubilization and sedimentation of the catalyst during the reaction, which causes various disadvantages such that: (1) the reaction rate decreases due to inactivation of the catalyst; (2) the sedimented copper salt is highly corrosive to corrode apparatuses; (3) the sedimented copper salt attaches to the wall of the reactor or pipe line, resulting in reduction of heat transfer efficiency or obstruction of the pass; (4) side reactions are induced; (5) the rate of recovery of the catalyst from the reaction mixture is reduced; and the like. These phenomena not only are disadvantageous in a batchwise reaction system but give rise to a momentous problem for stable operation particularly in a continuous reaction system.
The present inventors have conducted extensive investigations on a continuous process for preparing benzophenone-azines that can be effected on an industrial scale without involving the above-described disadvantages and, as a result, found that insolubilization of copper salt catalysts is primarily influenced by water present in the reaction system and can be suppressed by maintaining the water concentration in the reaction mixture at 1,000 ppm or less during the reaction in a continuous system [Japanese Patent Application (OPI) No. 103816/79 (the term "OPI" as used herein means an "unexamined published patent application")]. Several possible methods are considered in order to remove produced water from the reaction mixture to control the water concentration. From the standpoint of avoiding adverse influences on the reaction system and workability, the most practical process comprises introducing more oxygen or oxygen-containing gases than necessary for the oxidation reaction whereby produced water is driven out of the system together with the excessive oxygen. It is necessary to increase the amount of the gas to be introduced in order to ensure a further lowered water concentration. On the other hand, the oxidation reaction rate of benzophenone-imines depends on the oxygen partial pressure. Therefore, if in using inexpensive air as an oxygen-containing gas, the reaction is advantageously performed under pressure for attaining an industrially practical reaction rate in view of the low oxygen level in air. However, the so increased pressure rises the partial pressure of water even if the water concentration in the gaseous phase in the reaction system is low, thus resulting in rise of the water concentration in the liquid phase. Therefore, to elevate the reaction pressure requires increase of the amount of the gas to be introduced as compared with the reaction under a low pressure, which entails increase of cost of power.